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40 lines
1.8 KiB
TeX
40 lines
1.8 KiB
TeX
\pagebreak
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\section{The \code{taskloop} Construct}
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\label{sec:taskloop}
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The following example illustrates how to execute a long running task concurrently with tasks created
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with a \code{taskloop} directive for a loop having unbalanced amounts of work for its iterations.
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The \code{grainsize} clause specifies that each task is to execute at least 500 iterations of the loop.
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The \code{nogroup} clause removes the implicit taskgroup of the \code{taskloop} construct; the explicit \code{taskgroup} construct in the example ensures that the function is not exited before the long-running task and the loops have finished execution.
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\cexample[4.5]{taskloop}{1}
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\ffreeexample[4.5]{taskloop}{1}
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%\clearpage
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Because a \code{taskloop} construct encloses a loop, it is often incorrectly
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perceived as a worksharing construct (when it is directly nested in
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a \code{parallel} region).
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While a worksharing construct distributes the loop iterations across all threads in a team,
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the entire loop of a \code{taskloop} construct is executed by every thread of the team.
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In the example below the first taskloop occurs closely nested within
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a \code{parallel} region and the entire loop is executed by each of the \plc{T} threads;
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hence the reduction sum is executed \plc{T}*\plc{N} times.
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The loop of the second taskloop is within a \code{single} region and is executed
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by a single thread so that only \plc{N} reduction sums occur. (The other
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\plc{N}-1 threads of the \code{parallel} region will participate in executing the
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tasks. This is the common use case for the \code{taskloop} construct.)
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In the example, the code thus prints \code{x1 = 16384} (\plc{T}*\plc{N}) and
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\code{x2 = 1024} (\plc{N}).
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\cexample[4.5]{taskloop}{2}
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\ffreeexample[4.5]{taskloop}{2}
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